This invention relates to an electronic tube, more particularly, an electronic tube which operates under application of electric field and magnetic field.
In an electronic tube of this type, for example, a magnetron or a transmitting tube, an envelope forming a portion of the vacuum vessel is generally constituted by an electrode member on the high voltage side and a stem used to insulate an electrode member on the low voltage side from the high voltage electrode member. Generally, the stem is made of such an insulator as ceramic and is required to have electrical and magnetic insulating properties so as to hold the two electrode members at a definite spacing.
A prior art electronic tube shown in FIG. 1 comprises an inverted cup shaped electrode 1 made of copper, for example, and the lower end of the electrode 1 is soldered to the metallized surface 3a coated on the upper end of a cup shaped insulator 3 via a short cylindrical metal member 2 which is made of such a metal as Kovar (trade mark) having a small thermal expansion coefficient, thus forming an envelope. The metal member 2 having a low thermal expansion coefficient acts as a buffer against thermal deformation of the envelope. A rod shaped electrode 4 is disposed in the cup shaped electrode 1 and along the axis of the envelope. The electrode 4 is connected to a terminal 5 extending through the insulator 3. The lower end of the insulator 3 is formed with a metallized surface 3b which is brazed to the terminal 5 by means of a brazing metal 6 made of Kovar, for example. A magnetic coil 7 is provided about the cup shaped electrode 1 to generate magnetic field within the electrode 1.
In operation, when electric field and magnetic field are applied, in magnetic point of view, the magnetic flux flows in a direction shown by dotted lines a, and since Kovar utilized to form joints A and B is ferromagnetic, leakage flux tends to flow between the joints as shown by dotted lines b. In electrical point of view, the periphery of the metallized surface 3a is generally irregular, so that a large potential gradient will appear at this portion with the result that charged particles such as electrons and ions tend to emit from the joint A into the highly evacuated electronic tube when an intense electric field is applied. When the magnetic field is removed and hence the leakage flux b disappears, the charged particles emitted into the tube would migrate along the electric field shown by solid lines c towards electrode 1. At this time, since the travel of the charged particles is shorter than the means free path of the charged particles in the evacuated tube, there is no chance of collision between the charged particles and the gas molecules remaining in the tube. Accordingly, no ion is multiplied in the tube and glow discharge would not occur.
In the presence of the leakage flux b between joints A and B, however, the charged particles emitted from the joint A spirally moves toward the joint B along the equimagnetic potential planes so that their travel reaches a value that can not be neglected with respect to the mean free path, thus increasing the chance of collision against the molecules of the residual gas. Accordingly, ions are multiplied, thus resulting in glow discharge. Due to heat shock caused by the glow discharge, that is, the collision of the multiplied ions, the insulator 3 would be ruptured.